1. Field of the Inventive Concepts
The inventive concepts disclosed and claimed herein generally relate to microfluidic devices which have a plurality of channels for distributing portions of a fluid sample for analysis.
2. Brief Description of Related Art
Analysis of fluid samples is important in medical applications, such as the diagnosis of many diseases. In particular, the collection, processing, and analysis of blood specimens are a crucial aspect of many medical diagnoses for determining treatment options. Conventional blood analysis processes require that several sample tubes need to be drawn from the patient depending on the types of tests required (e.g., some assays require EDTA-treated plasma, heparin-treated plasma or whole blood). Commonly, the multiple blood samples are taken using color-coded blood collecting tubes so that the blood samples can be treated with the variety of different additives (reagents) therein for being subjected to different testing protocols. After being drawn the samples generally need to be centrifuged to separate the cells from the plasma. The correct sample types then need to be placed on an instrument for the desired assays.
Blood is a complex mixture of suspended cellular components (erythrocytes, leukocytes, and thrombocytes) and dissolved substances (e.g., electrolytes, proteins, lipids, carbohydrates, and amino acids). If blood is withdrawn from a vein and placed in a plain, untreated test tube, the blood will clot. Blood specimens of this type are used to harvest blood serum for testing, if a fresh blood specimen is promptly mixed with an anticoagulant (e.g., oxalates, citrates, ethylene diamine tetraacetates, heparins), the whole blood will remain fluid, allowing the cells to remain homogenously suspended in the blood plasma. If the anticoagulated whole blood specimen is allowed to stand for a sufficient length of time or the specimen is centrifuged, the cells will sediment to the bottom of the test tube, leaving a supernatant fluid called blood plasma. The principle difference between serum and plasma is that serum contains no fibrinogen. The fibrinogen is consumed in its conversion to an insoluble fibrin matrix which traps the cells and forms the clot. Therefore, an important early decision in blood collection (Vaught, J. B., “Blood Collection, Shipment, Processing, and Storage” Cancer Epidemiol Biomarkers Prev. September 2006 15:1582) is whether to collect anticoagulated blood (consisting of plasma, buffy coat, and red blood cells (RBCs)) or coagulated blood (consisting of serum and clotted RBCs).
There are several types of anticoagulants, which differ in their mechanisms of action and which need to be chosen carefully to avoid problems with certain laboratory applications. Heparin, for example, binds to and accelerates the inactivation of thrombin and other clotting factors. EDTA chelates metals, such as calcium and magnesium, which may be beneficial for some blood-based assays but which adversely affect others. As an anticoagulant, EDTA is well suited for DNA-based assays but is problematic for cytogenetic analysis. Despite anecdotal accounts of problems in PCR assays, studies have generally found that the use of heparin or EDTA produces equivalent results in PCR assays. Acid citrate dextrose (ACD) also chelates calcium. Citrate-stabilized blood results in better quality RNA and DNA than other anticoagulants and yields more lymphocytes for culture. However, in liquid form, acid citrate dextrose dilutes plasma, and a dilution factor will have to be considered when calculating assay results. If variable volumes of blood are drawn from study participants, dilution can result in erroneous results if volumes are not carefully recorded.
Other special collection tubes, such as serum separator tubes and CELL PREPARATION TUBES (Becton Dickinson), allow for more convenient separation of blood fractions. However, the serum separator tubes have been found to affect some assays, such as thyroxin and cortisol. Special collection tubes with protease inhibitors have been developed, which preserve proteins for proteomics analyses. The analysis of trace metals in blood also requires caution, as trace metals may be present in the evacuated collection tubes.
Although collection of multiple samples into multiple containers is the current state of the art in blood collection, such multiple sample collection is not only inconvenient for the subject who must provide the blood samples, but it also introduces potential for processing errors by loss or mislabeling of tubes, and variation in tube and/or reagent quality leading to spurious laboratory results. Furthermore, it is often difficult to obtain a sufficient amount of blood from a particular subject to dispose into the tube. Small, capillary tube samples are generally easier to obtain. A need exists for a device and method which allows for a more efficient treatment and analysis of fluid samples, such as blood samples, taken from a subject. It is to such a device and method that the inventive concepts disclosed and claimed herein are directed.